In the art of RFID, a coupling device for a transponder (such as a RFID transponder) generally comprises a primary antenna configured for long range communication with an external RFID reader and a secondary antenna connected in series, wherein the secondary antenna is configured as such to be inductively coupled to an antenna of the transponder device for example a chip or chip module. Such a coupling device is generally called a booster or a booster antenna.
It is noted that the term ‘chip’ and ‘chip module’ can be interchanged throughout this document as a chip module generally includes a chip.
Inductive coupling, also called magnetic, capacitive or reactive coupling, is defined in contrast to direct electrical coupling by electrically conductive material. References in the present description to inductive, magnetic, capacitive or reactive coupling refer to a coupling that is predominantly or primarily inductive, magnetic, capacitive or reactive. It will be appreciated that a coupling that is primarily inductive may also include some capacitive coupling. Conversely, a coupling that is primarily capacitive may also include some inductive (magnetic) coupling as a secondary coupling mechanism. Systems using primarily inductive coupling are referred to herein as inductive coupling, and systems using primarily capacitive coupling are referred to herein as capacitive coupling.
This technology has been extensively used for manufacturing non-contact cards. An example, is illustrated in U.S. Pat. No. 5,955,723. A transponder unit comprises a chip with an antenna and a booster antenna, comprising a primary antenna and a secondary antenna connected in series. All the elements are integrated in the card body and aim to extend the range and the quality of the communication of the transponder unit, in particular in the case of a non-contact card.
A similar approach for RFID smart card is described in EP 0 931 295. A module is provided with an antenna on the module, and the module is fixed in a recess of the card substrate accommodated with an inductive booster antenna. The mounting is achieved in such a way that the antenna on the module is inductive coupled with the secondary antenna of the booster. Similar modules with antenna are disclosed in EP 0 875 039 and WO 07 026 077. This solution is particularly interesting for dual (contact and non-contact) interface modules.
An alternative is presented in EP 0 977 145, where an antenna is formed directly on the surface of the chip, when the primary and secondary antennas of the coupling device are formed on a module board on which the chip is going to be mounted by the flip-chip method. This kind of small transponders (chip and antenna together) is called coil-on-chip and is known in the art and largely distributed on the market.
In all the documents of the prior art cited above, a booster working with a transponder unit is proposed. The chip is electrically connected to the larger antenna (the primary antenna of the booster or coupling device) without physical connection. Such units are much more resistant to mechanical stress than the ones using traditional connecting means such as pads, studs or wires. The antenna of the transponder is kept small (about the size of the chip or of the module) and is mounted on the same rigid structure as the chip. It can be the chip itself or in/on the chip packaging (as a chip module).
All configurations disclosed above, also as dipole, patch, slot, spiral, wire, single-loop, multi-loops and various hybrid antenna types are suitable for such inductive coupling systems. The mechanism for generating the magnetic field in the magnetic coupling device may vary based on the antennas type or configuration. All types of coupling, at low frequency (LF: 30-300 kHz), high frequency (HF: 3-30 MHz) or ultra-high frequency (UHF: over 300 MHz), are possible.
A key issue of this technology is to find a simple and effective mass production process to manufacture the booster. The problem is that all elements of the booster have to be tuned accurately in order to obtain the desired transmission characteristic and performance of the whole system (booster+transponder device). This is illustrated for example in the equation giving the mutual inductance in EP 1 325 468.
Many solutions are proposed today that all involve a multiple step manufacturing process.
One solution (used for example by Smart Packaging Solutions, France) is to use an etched antenna on a core sheet of dielectric material. Two extremities of the antenna, one on each side of the sheet, are enlarged to form the opposing electrode surface of a capacitor. By choosing accurately the two surface ratio, one can tune the capacitance of the element. A drawback of this method is that the antenna paths on each side of the sheet have to be connected electrically through the sheet.
Other solutions like connecting capacitors, closing antenna loops, etc . . . all imply additional manufacturing steps, most of the time with high technical difficulties and error potential. This complexity implies lower reproducible and quality yield that are essential drawbacks for mass production.
Further, it has been tried in the past to combine HF and UHF transmission modes on the same transponder (for example a card) and solutions have been developed in the art.
For example, FIG. 1 illustrates a first solution with two antennas and modules being placed side-by-side. On the top half of the transponder, an HF part of the transponder is illustrated with a chip or chip module 4 being electrically connected to an antenna 2.
On the bottom half of the transponder, an UHF part is illustrated with its booster antenna 3 coupled to a chip module 5 in an inductive manner, as discussed hereabove.
As one can easily see, this configuration of FIG. 1 is problematic as it covers the entire surface of the card and it is not possible to integrate other technologies in the card.
FIG. 2 illustrates a proposed solution in which the UHF antenna and transponder are placed inside of the area covered by the HF antenna. This configuration is interesting as about half of the card is now free but it has a substantive disadvantage: the HF antenna forms a faraday cage when the electrical circuit is closed and this drastically reduces the performance of the UHF part. The read distance is then only a couple of centimeters (up to max. 3 cm) which is neither practical nor acceptable.